1. Field of the Invention
The invention relates to the production of micro-accelerometers machined in silicon, in particular accelerometers for applications assisting with navigation in aircraft.
More precisely, the invention relates to a resonator accelerometer, in which a micromachined proof mass is connected by a vibrating beam, also micromachined, to a fixed frame forming a part of the framework of the accelerometer. The beam is mechanically tensioned by the weight of the proof mass, and the vibration of the beam is electrostatically excited by a tuned circuit comprising a moving-electrode capacitor, the position of the beam determining the position of the electrode. The electric field applied to the capacitor by the tuned circuit tends to move the beam, and the movement of the beam changes the value of the capacitor of the tuned circuit; the feedback of the tuned circuit is such that mechanical and electrical resonance occurs at a natural vibration frequency of the beam. The resonance frequency, that is to say the frequency at which the beam naturally enters into self-sustained vibration, depends on the mechanical tension longitudinally exerted on it, as is the situation, for example, with a musical instrument string. This mechanical tension itself depends on the acceleration to which the proof mass exerting the tension is subjected. A frequency measurement in the resonant circuit therefore represents an acceleration measurement.
Such a micro-accelerometer is therefore a combination of mechanical structure (proof mass, vibrating beam, other suspension arms, fixed framework) and electrical structure (capacitor electrodes, current-feed connections, and external circuitry forming a resonant circuit).
The characteristics expected of such an accelerometer are principally small size, good sensitivity along a well-identified axis of acceleration measurement, referred to as the sensitive axis, low sensitivity to accelerations along axes perpendicular to the sensitive axis, good linearity and good accuracy in the acceleration measurement, good mechanical strength both in the event of accelerations or impacts in the sense of the sensitive axis and in transverse directions, and lastly a low fabrication cost.
The cost is limited even further by the fact that batch fabrication can be used, which is why silicon-machining processes derived from integrated-circuit fabrication technologies have been envisaged.
2. Discussion of the Background
It has already been proposed, in particular, to produce both the proof mass and its suspension in silicon, the rest of the accelerometer being in quartz, which it is also known how to micromachine, electrical electrodes being deposited on the quartz such that they face towards the active silicon plate. Drawbacks have been observed with these hybrid structures, in particular because this makes it more difficult to produce the electrical parts of the resonator.
Silicon micro-accelerometer structures have also been proposed, in which the vibrating beam constitutes a deformable mechanical support carrying a strain gauge incorporated in a circuit capable of detecting resistance variations and amplifying them to inject them back into an electrostatic drive for the beam. This structure has the advantage of avoiding problems with electrical coupling between the elements which do not actually play a part in the measuring circuit, but fabrication is more complicated, especially because of the need for strain gauges and connections of the electrical circuit. In this case, only one of the plates is electrically active; the other plates are above all used as a closure cover. Errors are moreover induced by the use of heterogeneous materials.